This invention relates generally to control of a continuous casting process for minimization of solidification porosity and macrosegregation in the continuous strand thus produced and relates specifically to optimal operation of a wheel-type caster such that casting rate is substantially maximized subject to the foregoing condition.
Typically in traditional ingot casting, a solidification shrinkage cavity or pipe is formed upon complete solidification of the ingot extending from the top of the ingot down into the center of the ingot, especially when the molten alloy being cast is degassed, as is usual. This result is generally undesirable in that during subsequent rolling operations closure and welding of the cavity may be less than complete due to oxidation of the surface of the cavity.
By contrast, in continuous casting, formation of a shrinkage pipe is avoided due to the continuous feeding of molten alloy into the solidification zone, so long as feeding of molten alloy is sufficient for a given casting speed. However, if casting speed exceeds the feeding rate, then a continuous solidification pipe appears along the internal longitudinal axis of the strand.
The solidification zone or length in continuous casting may be conceptually visualized as a stationary converging tube or elongated cone extending into the advancing solidifying strand into which molten alloy continually flows and is progressively drawn off while flowing down the cone as the solidifying walls of the strand progressively build up. At the terminus or apex of the cone, the molten flow is exhausted upon solidification becoming complete at the point where the solidifying walls of the advancing strand merge. The length of this solidification zone will change with casting speed, e.g. as casting speed increases the zone is elongated. However, if casting speed is increased to the point where feeding is insufficient, there will occur feed starvation such that the solidification zone does not converge and an elongated void appears along th interval axis of the cast strand (i.e. the stream of molten alloy is depleted before "reaching" the solidification apex).
Even though continuous casting under proper operation eliminates the formation of a shrinkage pipe in the cast product, solidification porosity is typically found along the internal longitudinal axis of the strand. The term "solidification porosity" refers to the presence of discrete pores of macroscopic dimension internal to the cast strand. Such porosity is to be distinguished from pore formation arising from the evolution of dissolved gases upon solidification of molten alloy, which may be substantially eliminated by degassing the molten alloy prior to pouring. The presence of solidification porosity indicates the condition of discontinuous feeding occurring in the vicinity of the terminus of the solidification zone even though feed rate and casting speed are apparently matched, i.e. a nominal decrease in speed has no appreciable effect.
The occurrence of solidification porosity in a cast strand presents a two-fold disadvantage. First, the presence of porosity is objectionable per se in that closure & welding of the voids frequently is less than complete in subsequent rolling operations. Second, discontinuous feeding or feed starvation in the solidification terminus, as manifested by the occurrence of solidification porosity, promotes macrosegregation in the casting. The term "macrosegregation" is used in the conventional sense to refer to compositional inhomogeneity across the cast section resulting from progressive enrichment of the molten alloy building in advance of the solidification front within the solidifying strand.
A significant advantage of continuous casting over ingot casting is that macrosegregation is substantially diminished since the solute enriched molten core is continually mixed with fresh feed of molten alloy at nominal solute concentration, provided, however, that feeding is sufficient. Conversely, this advantage of continuous casting will not be fully realized if feeding is discontinuous as would be indicated by the presence of solidification porosity.
The present invention is especially concerned with wheel-type continuous casting (having a curvilinear mold) as opposed to linear-type continuous casting (having a linear mold). Typically, in wheel-type continuous casting, a stream of molten alloy is directed from a nozzle into a chill channel in the outer periphery of a vertically rotating casting wheel or ring, the channel being enclosed by a flexible band urged against and cooperatively rotating with the wheel periphery. The molten stream is continuously solidified within the channel into a cast strand which is correspondingly directed from an exit point in the channel to accumulating apparatus.
Specifically, the present invention is directed towards overcoming the aforementioned disadvantages associated with solidification porosity in a continuously cast strand by providing means to minimize or eliminate formation of solidification porosity in economic fashion. Restated, an optimum casting speed and cooling rate are determined such that casting speed in substantially maximized subject to the condition that solidification porosity is minimized or eliminated and macrosegregation is substantially minimized in the cast strand.